Method and apparatus for sensing, measurement or characterization of display elements integrated with the display drive scheme, and system and applications using the same

ABSTRACT

Methods and systems for electrical sensing, measurement and characterization of display elements are described. An embodiment includes integrating the electrical sensing, measurement and characterization with the display drive scheme. This embodiment allows for measurement of DC or operational hysteresis voltages and/or response times of interferometric modulator MEMS devices, for example, to be fully integrated with the display driver IC and/or the display drive scheme. Another embodiment allows these measurements to be performed and used without resulting in display artifacts visible to a human user. Another embodiment allows the measurement circuitry to be integrated with the display driver IC and/or the display drive scheme re-using several existing circuitry components and features, thus allowing for integration of the measurement method and its use relatively easily.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.61/027,727, filed on Feb. 11, 2008, entitled “METHOD AND APPARATUS FORSENSING, MEASUREMENT OR CHARACTERIZATION OF DISPLAY ELEMENTS INTEGRATEDWITH THE DISPLAY DRIVE SCHEME, AND SYSTEM AND APPLICATIONS USING THESAME,” the disclosure of which is hereby incorporated by reference inits entirety.

BACKGROUND

1. Field of the Invention

This invention relates to microelectromechanical systems. Moreparticularly, this invention relates to methods and apparatus forimproving the performance of microelectromechanical systems such asinterferometric modulators.

2. Description of the Related Art

Microelectromechanical systems (MEMS) include micro mechanical elements,actuators, and electronics. Micromechanical elements may be createdusing deposition, etching, and/or other micromachining processes thatetch away parts of substrates and/or deposited material layers or thatadd layers to form electrical and electromechanical devices. One type ofMEMS device is called an interferometric modulator. As used herein, theterm interferometric modulator or interferometric light modulator refersto a device that selectively absorbs and/or reflects light using theprinciples of optical interference. In certain embodiments, aninterferometric modulator may comprise a pair of conductive plates, oneor both of which may be transparent and/or reflective in whole or partand capable of relative motion upon application of an appropriateelectrical signal. In a particular embodiment, one plate may comprise astationary layer deposited on a substrate and the other plate maycomprise a metallic membrane separated from the stationary layer by anair gap. As described herein in more detail, the position of one platein relation to another can change the optical interference of lightincident on the interferometric modulator. Such devices have a widerange of applications, and it would be beneficial in the art to utilizeand/or modify the characteristics of these types of devices so thattheir features can be exploited in improving existing products andcreating new products that have not yet been developed.

The systems, methods, and devices described herein each have severalaspects, no single one of which is solely responsible for its desirableattributes. Without limiting the scope, prominent features will now bediscussed briefly. After considering this discussion, and particularlyafter reading the section entitled “Detailed Description of CertainEmbodiments” one will understand how the features described hereinprovide advantages over other display devices.

SUMMARY

One aspect is a method, including applying a signal between a firstelectrode and a second electrode of a display element, where the signalis at a level which results in the display element being in a first of aplurality of display states, applying a second signal between the twothe electrodes to transition the display element from one state toanother, measuring an electrical response of the display element inresponse to the applied second signal, identifying whether or not thedisplay element reached the desired final state based upon the measuredelectrical response, and adjusting the second signal if the displayelement did not reach the desired final state.

Another aspect is an apparatus, including drive circuitry configured toapply a signal between a first electrode and a second electrode of adisplay element, where the signal is at a level which results in thedisplay element being in a first of a plurality of display states,feedback circuitry configured to measure an electrical response of thedisplay element in response to the applied signal, and a processorconfigured control the drive circuitry, receive information indicativeof the measured electrical response, identify an error in operation ofthe display element based upon the measured electrical response, andadjust the drive signal of the display element in response to theidentified error.

Another aspect is a display device, including means for applying asignal between a first electrode and a second electrode of a displayelement, where the signal is at a level which results in the displayelement being in a first of a plurality of display states, means formeasuring an electrical response of the display element in response tothe applied signal, means for identifying an error in operation of thedisplay element based upon the measured electrical response, and foradjusting the signal in response to the identified error.

Another aspect is a display device including an array of interferometricmodulators, drive circuitry configured to apply a signal between a firstelectrode and a second electrode of a display element, where the signalis at a level that results in the display element being in a first of aplurality of display states, feedback circuitry configured to measure anelectrical response of the display element in response to the applieddrive voltage, a processor configured control the drive circuitry,receive information indicative of the measured electrical response,identify an error in operation of the display element based upon themeasured electrical response, and adjust the signal in response to theidentified error, and a memory device configured to communicate with theprocessor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view depicting a portion of one embodiment of aninterferometric modulator display in which a movable reflective layer ofa first interferometric modulator is in a relaxed position and a movablereflective layer of a second interferometric modulator is in an actuatedposition.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device incorporating a 3×3 interferometric modulator display.

FIG. 3 is a diagram of movable mirror position versus applied voltagefor one exemplary embodiment of an interferometric modulator of FIG. 1.

FIG. 4 is an illustration of a set of row and column voltages that maybe used to drive an interferometric modulator display.

FIG. 5A illustrates one exemplary frame of display data in the 3×3interferometric modulator display of FIG. 2.

FIG. 5B illustrates one exemplary timing diagram for row and columnsignals that may be used to write the frame of FIG. 5A.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa visual display device comprising a plurality of interferometricmodulators.

FIG. 7A is a cross section of the device of FIG. 1.

FIG. 7B is a cross section of an alternative embodiment of aninterferometric modulator.

FIG. 7C is a cross section of another alternative embodiment of aninterferometric modulator.

FIG. 7D is a cross section of yet another alternative embodiment of aninterferometric modulator.

FIG. 7E is a cross section of an additional alternative embodiment of aninterferometric modulator.

FIG. 8 is a block diagram illustrating an example system configured todrive a display array and measure an electrical response of selecteddisplay elements, such as the interferometric modulator display deviceof FIG. 2.

FIG. 9 is a block diagram illustrating another example of circuitry thatcan be used to measure an electrical response of selected displayelements via the same circuitry used to apply a stimulus to the selecteddisplay elements, such as in the interferometric modulator displaydevice of FIG. 2.

FIG. 10A is a flowchart illustrating an example of a method of driving adisplay element, such as, for example, the interferometric modulator asillustrated in FIG. 1, where a ramped drive voltage is used.

FIG. 10B is a flowchart illustrating a method of calibrating drivevoltages for driving a display element including determining a drivevoltage based on a desired operational characteristic of the displayelement.

FIG. 10C is a flowchart illustrating another method of calibrating drivevoltages for driving a display element including adjusting a drivevoltage based on identifying an error condition when driving the displayelement.

FIG. 11A is an illustration of an example of a ramped voltage waveformfor driving a display element.

FIG. 11B is an illustration of a sensed electrical response of drivecircuitry connected to the display element that may be used in themethods illustrated in FIGS. 10A and 10B.

FIG. 12 illustrates an example of a drive voltage waveform for driving adisplay element and a corresponding electrical response sensed in drivecircuitry connected to the display element, such as may be used in themethods illustrated in FIGS. 10A and 10B.

FIG. 13A illustrates an example of a drive voltage waveform andcorresponding electrical response indicative of proper actuation of adisplay element, such as may be used in the method illustrated in FIG.10C.

FIG. 13B illustrates an example of a drive voltage waveform andcorresponding electrical response indicative of an example of erroneousactuation of a display element, such as may be used in the methodillustrated in FIG. 10C.

FIG. 14 is a flowchart illustrating a method for driving a displayelement and measuring an electrical response of the display element todetermine a drive voltage to achieve a desired operationalcharacteristic, where the drive voltage results in a display statetransition that is substantially undetectable to human vision.

FIG. 15 illustrates an example of a drive voltage waveform andcorresponding sensed electrical response that may be used in the methodillustrated in FIG. 15.

FIG. 16A is a block diagram illustrating an example of circuitry fordriving an isolated portion of a display array and for sensing anelectrical response of the isolated area.

FIG. 16B illustrates an equivalent circuit illustrating the electricalrelationship of capacitance of a display area being sensed, andcapacitances of other display areas not being sensed.

DETAILED DESCRIPTION

The following detailed description is directed to certain specificembodiments. However, other embodiments may be used and some elementscan be embodied in a multitude of different ways. In this description,reference is made to the drawings wherein like parts are designated withlike numerals throughout. As will be apparent from the followingdescription, the embodiments may be implemented in any device that isconfigured to display an image, whether in motion (e.g., video) orstationary (e.g., still image), and whether textual or pictorial. Moreparticularly, it is contemplated that the embodiments may be implementedin or associated with a variety of electronic devices such as, but notlimited to, mobile telephones, wireless devices, personal dataassistants (PDAs), hand-held or portable computers, GPSreceivers/navigators, cameras, MP3 players, camcorders, game consoles,wrist watches, clocks, calculators, television monitors, flat paneldisplays, computer monitors, auto displays (e.g., odometer display,etc.), cockpit controls and/or displays, display of camera views (e.g.,display of a rear view camera in a vehicle), electronic photographs,electronic billboards or signs, projectors, architectural structures,packaging, and aesthetic structures (e.g., display of images on a pieceof jewelry). MEMS devices of similar structure to those described hereincan also be used in non-display applications such as in electronicswitching devices.

Methods and systems for electrical sensing, measurement andcharacterization of display elements are described. An embodimentincludes integrating the electrical sensing, measurement andcharacterization with the display drive scheme. This embodiment allowsfor measurement of DC or operational hysteresis voltages and/or responsetimes of interferometric modulator MEMS devices, for example, to befully integrated with the display driver IC and/or the display drivescheme. Another embodiment allows these measurements to be performed andused without resulting in display artifacts visible to a human user.Another embodiment allows the measurement circuitry to be integratedwith the display driver IC and/or the display drive scheme re-usingseveral existing circuitry components and features, thus allowing forintegration of the measurement method and its use relatively easily.

One interferometric modulator display embodiment comprising aninterferometric MEMS display element is illustrated in FIG. 1. In thesedevices, the pixels are in either a bright or dark state. In the bright(“on” or “open”) state, the display element reflects a large portion ofincident visible light to a user. When in the dark (“off” or “closed”)state, the display element reflects little incident visible light to theuser. Depending on the embodiment, the light reflectance properties ofthe “on” and “off” states may be reversed. MEMS pixels can be configuredto reflect predominantly at selected colors, allowing for a colordisplay in addition to black and white.

FIG. 1 is an isometric view depicting two adjacent pixels in a series ofpixels of a visual display, wherein each pixel comprises a MEMSinterferometric modulator. In some embodiments, an interferometricmodulator display comprises a row/column array of these interferometricmodulators. Each interferometric modulator includes a pair of reflectivelayers positioned at a variable and controllable distance from eachother to form a resonant optical gap with at least one variabledimension. In one embodiment, one of the reflective layers may be movedbetween two positions. In the first position, referred to herein as therelaxed position, the movable reflective layer is positioned at arelatively large distance from a fixed partially reflective layer. Inthe second position, referred to herein as the actuated position, themovable reflective layer is positioned more closely adjacent to thepartially reflective layer. Incident light that reflects from the twolayers interferes constructively or destructively depending on theposition of the movable reflective layer, producing either an overallreflective or non-reflective state for each pixel.

The depicted portion of the pixel array in FIG. 1 includes two adjacentinterferometric modulators 12 a and 12 b. In the interferometricmodulator 12 a on the left, a movable reflective layer 14 a isillustrated in a relaxed position at a predetermined distance from anoptical stack 16 a, which includes a partially reflective layer. In theinterferometric modulator 12 b on the right, the movable reflectivelayer 14 b is illustrated in an actuated position adjacent to theoptical stack 16 b.

The optical stacks 16 a and 16 b (collectively referred to as opticalstack 16), as referenced herein, typically comprise several fusedlayers, which can include an electrode layer, such as indium tin oxide(ITO), a partially reflective layer, such as chromium, and a transparentdielectric. The optical stack 16 is thus electrically conductive,partially transparent, and partially reflective, and may be fabricated,for example, by depositing one or more of the above layers onto atransparent substrate 20. The partially reflective layer can be formedfrom a variety of materials that are partially reflective such asvarious metals, semiconductors, and dielectrics. The partiallyreflective layer can be formed of one or more layers of materials, andeach of the layers can be formed of a single material or a combinationof materials.

In some embodiments, the layers of the optical stack 16 are patternedinto parallel strips, and may form row electrodes in a display device asdescribed further below. The movable reflective layers 14 a, 14 b may beformed as a series of parallel strips of a deposited metal layer orlayers (orthogonal to the row electrodes of 16 a, 16 b) deposited on topof posts 18 and an intervening sacrificial material deposited betweenthe posts 18. When the sacrificial material is etched away, the movablereflective layers 14 a, 14 b are separated from the optical stacks 16 a,16 b by a defined gap 19. A highly conductive and reflective materialsuch as aluminum may be used for the reflective layers 14, and thesestrips may form column electrodes in a display device.

With no applied voltage, the gap 19 remains between the movablereflective layer 14 a and optical stack 16 a, with the movablereflective layer 14 a in a mechanically relaxed state, as illustrated bythe pixel 12 a in FIG. 1. However, when a potential difference isapplied to a selected row and column, the capacitor formed at theintersection of the row and column electrodes at the corresponding pixelbecomes charged, and electrostatic forces pull the electrodes together.If the voltage is high enough, the movable reflective layer 14 isdeformed and is forced against the optical stack 16. A dielectric layer(not illustrated in this Figure) within the optical stack 16 may preventshorting and control the separation distance between layers 14 and 16,as illustrated by pixel 12 b on the right in FIG. 1. The behavior is thesame regardless of the polarity of the applied potential difference. Inthis way, row/column actuation that can control the reflective vs.non-reflective pixel states is analogous in many ways to that used inconventional LCD and other display technologies.

FIGS. 2 through 5B illustrate one exemplary process and system for usingan array of interferometric modulators in a display application.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device that may incorporate aspects of the invention. In theexemplary embodiment, the electronic device includes a processor 21which may be any general purpose single- or multi-chip microprocessorsuch as an ARM, Pentium®, Pentium II®, Pentium III®, Pentium IV®,Pentium® Pro, an 8051, a MIPS®, a Power PC®, an ALPHA®, or any specialpurpose microprocessor such as a digital signal processor,microcontroller, or a programmable gate array. As is conventional in theart, the processor 21 may be configured to execute one or more softwaremodules. In addition to executing an operating system, the processor maybe configured to execute one or more software applications, including aweb browser, a telephone application, an email program, or any othersoftware application.

In one embodiment, the processor 21 is also configured to communicatewith an array driver 22. In one embodiment, the array driver 22 includesa row driver circuit 24 and a column driver circuit 26 that providesignals to a display array or panel 30. The cross section of the arrayillustrated in FIG. 1 is shown by the lines 1-1 in FIG. 2. For MEMSinterferometric modulators, the row/column actuation protocol may takeadvantage of a hysteresis property of these devices illustrated in FIG.3. It may require, for example, a 10 volt potential difference to causea movable layer to deform from the relaxed state to the actuated state.However, when the voltage is reduced from that value, the movable layermaintains its state as the voltage drops back below 10 volts. In theexemplary embodiment of FIG. 3, the movable layer does not relaxcompletely until the voltage drops below 2 volts. Thus, there exists awindow of applied voltage, about 3 to 7 V in the example illustrated inFIG. 3, within which the device is stable in either the relaxed oractuated state. This is referred to herein as the “hysteresis window” or“stability window.” For a display array having the hysteresischaracteristics of FIG. 3, the row/column actuation protocol can bedesigned such that during row strobing, pixels in the strobed row thatare to be actuated are exposed to a voltage difference of about 10volts, and pixels that are to be relaxed are exposed to a voltagedifference of close to zero volts. After the strobe, the pixels areexposed to a steady state voltage difference of about 5 volts such thatthey remain in whatever state the row strobe put them in. After beingwritten, each pixel sees a potential difference within the “stabilitywindow” of 3-7 volts in this example. This feature makes the pixeldesign illustrated in FIG. 1 stable under the same applied voltageconditions in either an actuated or relaxed pre-existing state. Sinceeach pixel of the interferometric modulator, whether in the actuated orrelaxed state, is essentially a capacitor formed by the fixed and movingreflective layers, this stable state can be held at a voltage within thehysteresis window with almost no power dissipation. Essentially nocurrent flows into the pixel if the applied potential is fixed.

In typical applications, a display frame may be created by asserting theset of column electrodes in accordance with the desired set of actuatedpixels in the first row. A row pulse is then applied to the row 1electrode, actuating the pixels corresponding to the asserted columnlines. The asserted set of column electrodes is then changed tocorrespond to the desired set of actuated pixels in the second row. Apulse is then applied to the row 2 electrode, actuating the appropriatepixels in row 2 in accordance with the asserted column electrodes. Therow 1 pixels are unaffected by the row 2 pulse, and remain in the statethey were set to during the row 1 pulse. This may be repeated for theentire series of rows in a sequential fashion to produce the frame.Generally, the frames are refreshed and/or updated with new display databy continually repeating this process at some desired number of framesper second. A wide variety of protocols for driving row and columnelectrodes of pixel arrays to produce display frames are also well knownand may be used in conjunction with the present invention.

FIGS. 4, 5A, and 5B illustrate one possible actuation protocol forcreating a display frame on the 3×3 array of FIG. 2. FIG. 4 illustratesa possible set of column and row voltage levels that may be used forpixels exhibiting the hysteresis curves of FIG. 3. In the FIG. 4embodiment, actuating a pixel involves setting the appropriate column to−V_(bias), and the appropriate row to +ΔV, which may correspond to −5volts and +5 volts, respectively. Relaxing the pixel is accomplished bysetting the appropriate column to +V_(bias), and the appropriate row tothe same +ΔV, producing a zero volt potential difference across thepixel. In those rows where the row voltage is held at zero volts, thepixels are stable in whatever state they were originally in, regardlessof whether the column is at +V_(bias), or −V_(bias). As is alsoillustrated in FIG. 4, it will be appreciated that voltages of oppositepolarity than those described above can be used, e.g., actuating a pixelcan involve setting the appropriate column to +V_(bis), and theappropriate row to −ΔV. In this embodiment, releasing the pixel isaccomplished by setting the appropriate column to V_(bias), and theappropriate row to the same −ΔV, producing a zero volt potentialdifference across the pixel.

FIG. 5B is a timing diagram showing a series of row and column signalsapplied to the 3×3 array of FIG. 2 which will result in the displayarrangement illustrated in FIG. 5A, where actuated pixels arenon-reflective. Prior to writing the frame illustrated in FIG. 5A, thepixels can be in any state, and in this example, all the rows are at 0volts, and all the columns are at +5 volts. With these applied voltages,all pixels are stable in their existing actuated or relaxed states.

In the FIG. 5A frame, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) areactuated. To accomplish this, during a “line time” for row 1, columns 1and 2 are set to −5 volts, and column 3 is set to +5 volts. This doesnot change the state of any pixels, because all the pixels remain in the3-7 volt stability window. Row 1 is then strobed with a pulse that goesfrom 0, up to 5 volts, and back to zero. This actuates the (1,1) and(1,2) pixels and relaxes the (1,3) pixel. No other pixels in the arrayare affected. To set row 2 as desired, column 2 is set to −5 volts, andcolumms 1 and 3 are set to +5 volts. The same strobe applied to row 2will then actuate pixel (2,2) and relax pixels (2,1) and (2,3). Again,no other pixels of the array are affected. Row 3 is similarly set bysetting columns 2 and 3 to −5 volts, and column 1 to +5 volts. The row 3strobe sets the row 3 pixels as shown in FIG. 5A. After writing theframe, the row potentials are zero, and the column potentials can remainat either +5 or −5 volts, and the display is then stable in thearrangement of FIG. 5A. It will be appreciated that the same procedurecan be employed for arrays of dozens or hundreds of rows and columns. Itwill also be appreciated that the timing, sequence, and levels ofvoltages used to perform row and column actuation can be varied widelywithin the general principles outlined above, and the above example isexemplary only, and any actuation voltage method can be used with thesystems and methods described herein.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa display device 40. The display device 40 can be, for example, acellular or mobile telephone. However, the same components of displaydevice 40 or slight variations thereof are also illustrative of varioustypes of display devices such as televisions and portable media players.

The display device 40 includes a housing 41, a display 30, an antenna43, a speaker 45, an input device 48, and a microphone 46. The housing41 is generally formed from any of a variety of manufacturing processesas are well known to those of skill in the art, including injectionmolding and vacuum forming. In addition, the housing 41 may be made fromany of a variety of materials, including, but not limited to, plastic,metal, glass, rubber, and ceramic, or a combination thereof. In oneembodiment, the housing 41 includes removable portions (not shown) thatmay be interchanged with other removable portions of different color, orcontaining different logos, pictures, or symbols.

The display 30 of exemplary display device 40 may be any of a variety ofdisplays, including a bi-stable display, as described herein. In otherembodiments, the display 30 includes a flat-panel display, such asplasma, EL, OLED, STN LCD, or TFT LCD as described above, or anon-flat-panel display, such as a CRT or other tube device, as is wellknown to those of skill in the art. However, for purposes of describingthe present embodiment, the display 30 includes an interferometricmodulator display, as described herein.

The components of one embodiment of exemplary display device 40 areschematically illustrated in FIG. 6B. The illustrated exemplary displaydevice 40 includes a housing 41 and can include additional components atleast partially enclosed therein. For example, in one embodiment, theexemplary display device 40 includes a network interface 27 thatincludes an antenna 43, which is coupled to a transceiver 47. Thetransceiver 47 is connected to a processor 21, which is connected toconditioning hardware 52. The conditioning hardware 52 may be configuredto condition a signal (e.g., filter a signal). The conditioning hardware52 is connected to a speaker 45 and a microphone 46. The processor 21 isalso connected to an input device 48 and a driver controller 29. Thedriver controller 29 is coupled to a frame buffer 28 and to an arraydriver 22, which in turn is coupled to a display array 30. A powersupply 50 provides power to all components as required by the particularexemplary display device 40 design.

The network interface 27 includes the antenna 43 and the transceiver 47so that the exemplary display device 40 can communicate with one or moredevices over a network. In one embodiment, the network interface 27 mayalso have some processing capabilities to relieve requirements of theprocessor 21. The antenna 43 is any antenna known to those of skill inthe art for transmitting and receiving signals. In one embodiment, theantenna transmits and receives RF signals according to the IEEE 802.11standard, including IEEE 802.11(a), (b), or (g). In another embodiment,the antenna transmits and receives RF signals according to the BLUETOOTHstandard. In the case of a cellular telephone, the antenna is designedto receive CDMA, GSM, AMPS, or other known signals that are used tocommunicate within a wireless cell phone network. The transceiver 47pre-processes the signals received from the antenna 43 so that they maybe received by and further manipulated by the processor 21. Thetransceiver 47 also processes signals received from the processor 21 sothat they may be transmitted from the exemplary display device 40 viathe antenna 43.

In an alternative embodiment, the transceiver 47 can be replaced by areceiver. In yet another alternative embodiment, network interface 27can be replaced by an image source, which can store or generate imagedata to be sent to the processor 21. For example, the image source canbe a digital video disc (DVD) or a hard-disc drive that contains imagedata, or a software module that generates image data.

Processor 21 generally controls the overall operation of the exemplarydisplay device 40. The processor 21 receives data, such as compressedimage data from the network interface 27 or an image source, andprocesses the data into raw image data or into a format that is readilyprocessed into raw image data. The processor 21 then sends the processeddata to the driver controller 29 or to frame buffer 28 for storage. Rawdata typically refers to the information that identifies the imagecharacteristics at each location within an image. For example, suchimage characteristics can include color, saturation, and gray-scalelevel.

In one embodiment the processor 21 includes a microcontroller, CPU, orlogic unit to control operation of the exemplary display device 40.Conditioning hardware 52 generally includes amplifiers and filters fortransmitting signals to the speaker 45, and for receiving signals fromthe microphone 46. Conditioning hardware 52 may be discrete componentswithin the exemplary display device 40, or may be incorporated withinthe processor 21 or other components.

The driver controller 29 takes the raw image data generated by theprocessor 21 either directly from the processor 21 or from the framebuffer 28 and reformats the raw image data appropriately for high speedtransmission to the array driver 22. Specifically, the driver controller29 reformats the raw image data into a data flow having a raster-likeformat, such that it has a time order suitable for scanning across thedisplay array 30. Then the driver controller 29 sends the formattedinformation to the array driver 22. Although a driver controller 29,such as a LCD controller, is often associated with the system processor21 as a stand-alone Integrated Circuit (IC), such controllers may beimplemented in many ways. They may be embedded in the processor 21 ashardware, embedded in the processor 21 as software, or fully integratedin hardware with the array driver 22.

Typically, the array driver 22 receives the formatted information fromthe driver controller 29 and reformats the video data into a parallelset of waveforms that are applied many times per second to the hundredsand sometimes thousands of leads coming from the display's x-y matrix ofpixels.

In one embodiment, the driver controller 29, array driver 22, anddisplay array 30 are appropriate for any of the types of displaysdescribed herein. For example, in one embodiment, driver controller 29is a conventional display controller or a bi-stable display controller(e.g., an interferometric modulator controller). In another embodiment,array driver 22 is a conventional driver or a bi-stable display driver(e.g., an interferometric modulator display). In one embodiment, adriver controller 29 is integrated with the array driver 22. Such anembodiment is common in highly integrated systems such as cellularphones, watches, and other small area displays. In yet anotherembodiment, display array 30 is a typical display array or a bi-stabledisplay array (e.g., a display including an array of interferometricmodulators).

The input device 48 allows a user to control the operation of theexemplary display device 40. In one embodiment, input device 48 includesa keypad, such as a QWERTY keyboard or a telephone keypad, a button, aswitch, a touch-sensitive screen, or a pressure- or heat-sensitivemembrane. In one embodiment, the microphone 46 is an input device forthe exemplary display device 40. When the microphone 46 is used to inputdata to the device, voice commands may be provided by a user forcontrolling operations of the exemplary display device 40.

Power supply 50 can include a variety of energy storage devices as arewell known in the art. For example, in one embodiment, power supply 50is a rechargeable battery, such as a nickel-cadmium battery or a lithiumion battery. In another embodiment, power supply 50 is a renewableenergy source, a capacitor, or a solar cell including a plastic solarcell, and solar-cell paint. In another embodiment, power supply 50 isconfigured to receive power from a wall outlet.

In some embodiments, control programmability resides, as describedabove, in a driver controller which can be located in several places inthe electronic display system. In some embodiments, controlprogrammability resides in the array driver 22. Those of skill in theart will recognize that the above-described optimizations may beimplemented in any number of hardware and/or software components and invarious configurations.

The details of the structure of interferometric modulators that operatein accordance with the principles set forth above may vary widely. Forexample, FIGS. 7A-7E illustrate five different embodiments of themovable reflective layer 14 and its supporting structures. FIG. 7A is across section of the embodiment of FIG. 1, where a strip of metalmaterial 14 is deposited on orthogonally extending supports 18. In FIG.7B, the moveable reflective layer 14 is attached to supports at thecorners only, on tethers 32. In FIG. 7C, the moveable reflective layer14 is suspended from a deformable layer 34, which may comprise aflexible metal. The deformable layer 34 connects, directly orindirectly, to the substrate 20 around the perimeter of the deformablelayer 34. These connections are herein referred to as support posts. Theembodiment illustrated in FIG. 7D has support post plugs 42 upon whichthe deformable layer 34 rests. The movable reflective layer 14 remainssuspended over the gap, as in FIGS. 7A-7C, but the deformable layer 34does not form the support posts by filling holes between the deformablelayer 34 and the optical stack 16. Rather, the support posts are formedof a planarization material, which is used to form support post plugs42. The embodiment illustrated in FIG. 7E is based on the embodimentshown in FIG. 7D, but may also be adapted to work with any of theembodiments illustrated in FIGS. 7A-7C, as well as additionalembodiments not shown. In the embodiment shown in FIG. 7E, an extralayer of metal or other conductive material has been used to form a busstructure 44. This allows signal routing along the back of theinterferometric modulators, eliminating a number of electrodes that mayotherwise have had to be formed on the substrate 20.

In embodiments such as those shown in FIG. 7, the interferometricmodulators function as direct-view devices, in which images are viewedfrom the front side of the transparent substrate 20, the side oppositeto that upon which the modulator is arranged. In these embodiments, thereflective layer 14 optically shields the portions of theinterferometric modulator on the side of the reflective layer oppositethe substrate 20, including the deformable layer 34. This allows theshielded areas to be configured and operated upon without negativelyaffecting the image quality. Such shielding allows the bus structure 44in FIG. 7E, which provides the ability to separate the opticalproperties of the modulator from the electromechanical properties of themodulator, such as addressing and the movements that result from thataddressing. This separable modulator architecture allows the structuraldesign and materials used for the electromechanical aspects and theoptical aspects of the modulator to be selected and to functionindependently of each other. Moreover, the embodiments shown in FIGS.7C-7E have additional benefits deriving from the decoupling of theoptical properties of the reflective layer 14 from its mechanicalproperties, which are carried out by the deformable layer 34. Thisallows the structural design and materials used for the reflective layer14 to be optimized with respect to the optical properties, and thestructural design and materials used for the deformable layer 34 to beoptimized with respect to desired mechanical properties.

The following description is directed to methods and devices used toprovide, monitor and adapt drive voltages of a wide variety of MEMSelements, such as MEMS switches, and other elements having deflected ordeformed electrodes and/or mirrors. Although the specific examplesdiscussed use interferometric modulators as the elements, the principlesdiscussed may apply to other MEMS elements as well.

Display devices like those based on interferometric modulator technologymay be measured and characterized electronically and/or mechanically.Depending on the display technology, these measurements can form a partof calibration of the display module (the display “module” referred toherein includes the display panel, the display driver, and associatedcomponents such as cabling, etc.), and the measurement parameters may bestored into a non-volatile memory (e.g., NVRAM) in the display modulefor future use. As discussed above with reference to FIG. 3, theinterferometric modulators operate based on a potential differenceapplied to them. FIG. 3 shows that the interferometric modulators are ineither the relaxed (or released) state or in the actuated state,depending on the magnitude of the potential difference applied betweentheir electrodes. As shown, the changing of one state to another happensaccording to a hysteretic characteristic with a stability (or hold)window, where the device holds its current state when the appliedpotential difference falls within the hold window. As used herein, a“bias voltage” refers to a potential difference that falls within thehold window. Accordingly, as shown in FIG. 3, there are five inputvoltage difference ranges in some embodiments. Each of the five voltagedifference ranges has a title reflecting its effect on the state of theinterferometric modulator. Starting from the left of FIG. 3, the fivevoltage difference ranges are: 1) negative actuate (“Actuated”); 2)negative hold (“Stability Window”); 3) release (“Relaxed”); 4) positivehold (“Stability Window”); and 5) positive actuate (“Actuated”).

Based on theoretical understanding of the devices and past experimentalresults, approximate values of the thresholds between these inputvoltage difference ranges may be known, but in order to more optimallyoperate the interferometric modulator array, the threshold voltages canbe measured with more precision. For example, as described furtherherein, the thresholds may vary from device to device, lot to lot, overtemperature, and/or as the device ages. Threshold values may accordinglybe measured for each manufactured device or group of devices. One methodof measuring the threshold voltages is to apply inputs of variousvoltage differences while monitoring the state of the interferometricmodulators through observation of the optical characteristics of theinterferometric modulators. This may be accomplished, for example,through human observation or by use of an optical measurement device.Additionally or alternatively, the state of the interferometricmodulators may be monitored through electronic response measurement. Insome embodiments, the array driver 22 of the display array 30, discussedabove, may be configured to measure electrical responses of displayelements in order to determine the state and/or operationalcharacteristics of the display elements according to the methodsdiscussed below.

Often times, the behavior of a display device changes with the age ofthe display device, with variations in temperature of the display, withthe content of the images being displayed, etc. Display devices may haveone or more electrical parameters that change in relation to the opticalresponse or optical state. As discussed above, the interferometricmodulator is set to an actuated state when the electrostatic attractionbetween the reflective layer and the optical stack is great enough toovercome the mechanical restorative forces working to hold thereflective layer in the relaxed state. Because the reflective layer, theoptical stack, and the gap between them form two conductive platesseparated by a dielectric, the structure has a capacitance. Also,because the capacitance of the structure varies according to thedistance between the two plates, the capacitance of the structure variesaccording to the state of the interferometric modulator. Therefore, anindication of the capacitance can be used to determine the state of theinterferometric modulator.

In one aspect, an indication of the capacitance can be obtained, forexample, by sensing the current or charge used to change the voltageapplied between the reflective layer and the optical stack. A relativelyhigh amount of current or charge indicates that the capacitance isrelatively large. Similarly, a relatively low amount of current orcharge indicates that the capacitance is relatively small. The sensingof current or charge may be accomplished, for example through analog ordigital integration of a signal representing the charge or current.

Similar characteristics can apply to LCD display technology where thecapacitance of the device is related to the resulting optical brightnessof the cell at a certain temperature. In addition to the operationalcharacteristics of display element possibly changing with age, theoperational characteristics can be affected by the temperature of thedisplay elements. The temperature of a display element can depend on thepast optical response states that were displayed, and, thus, theoperational characteristics could vary independently for each displayelement in the display array of the display device.

In one embodiment, the relevant characteristics of the display device,like hysteresis voltages and response times for interferometricmodulator MEMS devices and brightness-voltage relationship for LCDdevices, are measured after manufacturing at the factory during acalibration procedure. This information can then be stored in a memorythe display module used for driving the display device. Since thecharacteristics of the display device may also change with temperatureand aging, for example, the effects of temperature and aging on thesecharacteristics (e.g., temperature coefficient) may be studied, measuredand also hardwired or stored in the memory of the display module. Inspite of this post-manufacturing characterization, however, thecalibration margins built into the display device may not allow forunpredictable changes in the characteristics of the display device. Insome cases, the lifetime and quality of a display device may be improvedby performing recalibration of the device after a certain period of use(e.g., one year), on a random length periodic basis, based on changes intemperature, etc. In other cases, the drive scheme may be robust enoughto compensate for changes in characteristics of the display devicewithout such recalibration. Examples of such recalibration and robustdrive schemes are discussed below.

FIG. 8 is a block diagram illustrating an example system 100 configuredto drive a display array 102 and measure an electrical response ofselected display elements, such as the interferometric modulators 12 aand 12 b of FIG. 1. The display array 102 comprises m columns by n rowsof N-component pixels (e.g., N may be 3 display elements including red,green and blue, for example). The system 100 further includes a columndriver comprising 2 or more digital to analog converters (DACs) 104 forsupplying two or more drive voltage levels as well as a column switchsubsystem 106 for selecting the columns to which data signals aresupplied. The system 100 further includes a row driver circuitcomprising two or more DACs 108 for supplying two or more drive voltagelevels as well as a row switch circuit 110 for selecting which row tostrobe. Note that the row and column drivers that are directly connectedto the display array in this schematic are shown as composed ofswitches, but several methods discussed below are applicable toalternative driver designs including a full analog display driver. Notethat while drive voltages are discussed herein, other drive signals,such as drive currents or drive charges may be used.

The row and column driver circuitry including the DACs 104 and 108 andthe switches 106 and 110 are controlled by digital logic of an arraydriver 112. As discussed above in reference to FIGS. 2 and 3, therow/column actuation protocol contained in the digital logic of thearray driver 112 may take advantage of a hysteresis property ofinterferometric modulator MEMS devices. For example, in a display arraycomprising interferometric modulators 12 having the hysteresischaracteristics of FIG. 3, the row/column actuation protocol can bedesigned such that during row strobing, display elements in the strobedrow that are to be actuated are exposed to an actuation voltagedifference (e.g., about 10 volts), and display elements that are to berelaxed are exposed to a voltage difference of close to zero volts, asshown in FIGS. 4 to 5. After the strobe, the display elements areexposed to a steady state voltage difference known as the bias voltage(e.g., about 5 volts) such that they remain in whatever state the rowstrobe last put them. After being written, each display element sees apotential difference within the “stability window” of 3-7 volts in thisexample. However, as discussed above, the characteristics of the displayelements may change with time and/or temperature or may respond morequickly or slowly to different drive voltage levels. As such, the arraydriver 112 and the DACs 104 and 108 may be configured to supply variablevoltage levels, depending on the embodiment.

In addition to the drive circuitry discussed above (including the DACs104 and 108 and the switches 106 and 110, and the array driver 112), theremaining blocks of the system 100 are able to apply further electricalstimulus to selected display elements, as well as to be able to measurethe electrical response of selected display elements in the displayarray 102. In this example, digital-to-analog converters (DACs) 114 and116 supply additional voltages to the display array 102 via the columnand row switches 106 and 110, respectively. In general, these mayrepresent internal or external voltage supply inputs to the row andcolumn drive circuitry.

In this example, a direct-digital-synthesis (DDS1) block 118 is used togenerate the electrical voltage stimulus that is added on the top of thevoltage level produced by the DAC 114 connected to the column switch106. The stimulus signal produced by the DDS1 block 118 may be producedby several alternative means such as an electrical oscillator, asaw-tooth waveform generator, etc. which are familiar to those skilledin the art. In various embodiments, the stimulus may be current orcharge, or even a controlled output impedance.

In the example shown in FIG. 8, the electrical response of the displayarray 102 is measured in the form of electrical current flowing throughthe display array 102 resulting from application of the electricalvoltage stimulus to the row and/or column electrodes via the row and/orcolumn switches 106 and 110, respectively. Other forms of measuredelectrical response can include voltage variations, etc. Atrans-impedance amplifier 120 (shown in FIG. 8 as a resistor 120Afollowed by an amplifier 120B) may be used to measure the electricalresponse. The display element(s) for which the measured electricalresponse corresponds depends on the states of the column and rowswitches 106 and 110. In alternative embodiments, analog, digital, ormixed-signal processing may be used for the purpose of measurement ofthe electrical response of the display array 102.

In one embodiment, the electrical response of a display element ismeasured directly by measuring the current through the input terminalsof the trans-impedance amplifier 120. In this embodiment, the profileand/or peak values, or other characteristics known to skilledtechnologists, can be used to identify certain operationalcharacteristics of the display element.

In another embodiment, operational characteristics of the displayelement being measured can be characterized by additional postprocessing of the electrical response output from the trans-impedanceamplifier 120. An example of using post processing techniques tocharacterize the capacitance and the resistive component of theimpedance of an interferometric modulator using the circuitry of FIG. 8is now discussed.

Since an interferometric modulator can be considered a capacitor, aperiodic stimulus, such as that which could be applied using the DDS1118, will result in a periodic output electrical response with a 90°phase lag. For example, the DDS1 118 could apply a sinusoidal voltagewaveform, sin(wt), to the column electrode of the display element. Foran ideal capacitor, the electrical response of the display element wouldbe a time derivative of the applied stimulus, or cos(wt). Thus, theoutput of the trans-impedance amplifier 120 would also be a cosinefunction. A second DDS, DDS2 122, applies a cosine voltage waveform thatis multiplied by the output of the trans-impedance amplifier 120 atmultiplier 124. The result is a waveform with a constant component and aperiodic component. The constant component of the output of themultiplier 124 is proportional to the capacitance of the displayelement. A filter 126 is used to filter out the periodic component andresult in an electrical signal that is used to characterize thecapacitance, and therefore the actuated or unactuated state, of thedisplay element.

For a display element that is an ideal capacitor, the output of thetrans-impedance amplifier 120 is a pure cosine function for the examplewhere the applied stimulus is a sine function. However, if the displayelement exhibits any non-capacitive impedance, due to leakage forexample, the output of the trans-impedance amplifier 120 will alsocontain a sine component. This sine component does not affect themeasurement of the capacitance, since it will be filtered out by thefilter 126. The sine component can be detected and used to characterizethe resistive portion of the impedance of the display element.

A periodic voltage waveform similar to the stimulus applied by the DDS1,sin(wt) for example, is multiplied by the output of the trans-impedanceamplifier 120 at a multiplier 128. The result is an electrical responsethat includes a constant component and a periodic component. Theconstant component is proportional to the resistive portion of theimpedance of the display element being measured. A filter 130 is used toremove the periodic component resulting in a signal that can be used tocharacterize the resistive portion of the impedance of the displayelement.

The outputs of the filters are converted to the digital domain by use ofa dual analog to digital converter (ADC) 132. The output of the dual ADC132 is received by the array driver 112 for use in carrying out themethods discussed below.

In the example circuitry shown in FIG. 8, the characterization stimulusis applied to a column electrode and the electrical response is measuredvia a row electrode. In other embodiments, the electrical response canbe measured from the same electrode, row or column, for example, towhich the stimulus is applied. FIG. 9 is a block diagram illustrating anexample of circuitry 150 that can be used to measure an electricalresponse of selected display elements via the same circuitry used toapply a stimulus to the selected display elements, such as in theinterferometric modulator display device of FIG. 2. The circuit 150comprises transistors N1 and P1 which mirror the current from thecurrent source transistors N2 and P2 used to drive the V_(out) signalapplied to the display element. Accordingly, the current I_(out) issubstantially equal to the current used for driving the V_(out) signal.Measuring the electrical response of the I_(out) signal may, therefore,be used to determine operational characteristics of the interferometricmodulators such as whether the interferometric modulators are in a highor low capacitance state. Other circuits may also be used. The circuit150 shown in FIG. 9 is applicable to alternative driver IC designs ordrive schemes for supplying a voltage waveform V_(out). The circuit 150depicted in the schematic of FIG. 9 can be used in current conveyorcircuits and in current feedback amplifiers, and can apply an electricalvoltage stimulus to the display array area and simultaneously replicatethe current (response) to a different pin (I_(out)) for purposes ofelectrical sensing.

There are several ways in which measured electrical responses, such asthose sensed by the systems shown in FIGS. 8 and 9, can be used as afeedback signal to affect the operation of the display driver circuitry.For example, the measured information may be analyzed in the digitaldomain, e.g., using the digital logic of array driver 112 and/or aprocessor configured to control the array driver 112 (e.g., theprocessor 21 and array driver 22 shown in FIG. 2) and then used toadaptively drive the display array 102. The measured electricalresponses may also be used to complete a feedback loop in the analogdomain (e.g., using the outputs of the DACs 104, 114, 108 and/or 116, orusing the output of the DDS1 118 shown in FIG. 8). Examples of methodsof driving interferometric modulator display elements using measuredelectrical responses as feedback are illustrated in FIGS. 10A-10C.

FIG. 10A is a flowchart illustrating an example of a method 200A ofdriving a display element, such as, for example, the interferometricmodulator as illustrated in FIG. 1, where a ramped drive voltage isused. In one embodiment, the method 200A can be performed by the arraydriver 112 for controlling the drive circuitry (e.g., the DACs 104, 108and 114, the switches 106 and 110, and the DSS1 118) shown in FIG. 8 todisplay images on the display array 102. In other embodiments, aprocessor such as the processor 21 in FIG. 2, can perform the method200A. The method 200A provides a method of adapting drive voltage levelsby applying a gradually increasing or decreasing voltage waveform to adisplay element and discontinuing the application of the voltagewaveform when a change in state of the display element is sensed. Inthis way, the applied voltages, including drive voltages to actuate orrelease the display element, can be changed only as much as necessary,thereby conserving power.

The method 200A starts at block 202 where the array driver 112 applies adrive voltage between a first electrode and a second electrode of adisplay element. The first electrode may be one of the movablereflective layers (column electrodes) 14 and the second electrode may beone of the row electrodes 16 of the interferometric modulators 12illustrated in FIG. 1. The drive voltage applied at block 202 may be avoltage at the bias voltage within the hysteresis window (e.g., 3-7volts as discussed above), or, alternatively may be a static voltagelevel outside of the hysteresis window. As used herein, a static voltageis a voltage that is non-varying over time, such as over an actuationperiod. The static drive voltage difference applied to the twoelectrodes at block 202 may be supplied by one or more of the DACs 104or 108 (FIG. 8) to the column and/or row electrodes respectively.

After the initial drive voltage is applied at the block 202, the method200A continues at block 204, where the array driver 112 ramps the levelof the drive voltage from a first level (e.g., the static voltage levelapplied at block 202) to a second level. FIG. 11A is an illustration ofan example of a ramped voltage waveform for driving a display elementthat may be used in the method 200A. In FIG. 11A, the initial drivevoltage applied at the block 202 is a 5 volt bias voltage 302 (thestatic voltage applied in block 202). At approximately 2 ms, a rampedvoltage waveform 304 is applied at block 204 in the method 200A. Theramped voltage waveform 304 continues to be increased until a measuredelectrical response, as sensed by electrical sensing feedback circuitrysuch as the trans-impedance amplifier 120 in FIG. 8, monitors anelectrical response of the display element at block 206. For example,the trans-impedance amplifier 120 may sense a change in the current toor from the display element, indicating a change in state of the displayelement.

In this example, the monitored electrical response is indicative of achange of state of the interferometric modulators 12 of FIG. 1. FIG. 11Bis an illustration of a sensed electrical response that may be sensedwith the electrical sensing feedback circuitry connected to the drivecircuitry of the display element using the method 200A illustrated inFIG. 10A. At about 4 ms, the sensed electrical current shows a sharprise 306 to a level of about +5 milliamps. The sensitivity of theamplifier to the sensed electrical current can depend on the resistanceof the circuitry being used for sensing. For example, in an embodimentsuch as that shown in FIG. 8, the resistance of the resistor 120A may bechosen to result in a output amplitude that is easily measurable,depending on the feedback circuitry. Upon detecting the rise 306 in thesensed current in block 206, the method 200A continues to block 208,where the ramped voltage waveform is discontinued as shown at 308 inFIG. 11A and reduced to the static (bias) voltage level of 5 volts at310 to allow the interferometric modulator to remain in the actuatedstate. In the example shown in FIG. 11A, the ramped voltage results inactuation of the display element at about 6 volts. This is merely anexample actuation level and other levels of voltage may result inactuation, depending on the design of the display element.

Although described above with respect to an actuation signal, a releasesignal can also be applied by the array driver 112 at the block 202 ofthe method 200A. For example, as shown in FIG. 11A at about 6 ms, arelease procedure is initiated and a ramped voltage waveform 312 isapplied. The ramped voltage 312, applied at the block 204 of method 200Areduces the drive voltage from the initial 5 volts (that was applied atthe block 202) to about 4 volts. When the ramped voltage waveformreaches about 4 volts, interferometric modulator 12 releases and theelectrical sensing circuitry measures a sharp decline 314 in the sensedcurrent (sensed at the block 206) to a level of about −3 milliamps,indicating that the display element has released. Upon sensing thedecline in current at 314 due to the change in IMOD state, the method200A continues to block 208, where the ramped drive voltage waveform isdiscontinued and the drive voltage is reduced (see 316) to the 5 voltbias voltage level at 318 such that the display element remains in thereleased state. Once again, the voltage and current levels shown in FIG.11 are exemplary only, and other levels may be indicative of actuationand or release of a display element. The ramped voltage waveform appliedat the block 204 may be applied using the DDS1 118 illustrated in FIG.8.

In some embodiments, the rate of increase or decrease of the rampedvoltage waveform is at a predetermined rate that is slow relative to theresponse time of the display element when an actuation and/or releaseevent occurs. In this way, the change in voltage levels from the biaslevel to the actuation and/or release voltage levels can be minimized.In another embodiment, the rate of increase and/or decrease in theramped voltage waveform is calibrated and chosen in order to achieve adesired operational characteristic of the display element, such as, forexample response time.

FIG. 10B is a flowchart illustrating a method 200B of calibrating drivevoltages for driving a display element. In one embodiment, the method200B can be used to determine an operational threshold drive voltagebased on a desired operational characteristic of the display element,e.g., response time. The method 200B includes a calibration portion,blocks 220 to 234, which, in one embodiment, can be performed at thetime of manufacture of the display element for initial calibration. Inthis embodiment, the process 200B can be performed by an externalprocessor connected to the display array, such as a test stand, forexample.

In another embodiment, the calibration blocks 220 to 234 can also beincluded in logic coupled to the display array so that the calibrationcan be performed at other times in order to recalibrate the displayelement. For example, the recalibration may be done on a periodic basisbased on the age of the display element, on a pseudo-random basis, basedon temperature, etc. In this embodiment, the method 200B can beperformed using the array driver 112 for controlling the drive circuitry(e.g., the DACs 104, 108 and 114, the switches 106 and 110, and the DDS1118) shown in FIG. 8 to display images on the display array 102. Inother embodiments, a processor such as the processor 21 in FIG. 2 canperform the method 200A. After calibration, the array driver 112 maydetermine a drive voltage (e.g., an initial drive voltage level and/or aramped voltage rate) in order to achieve a desired operationalcharacteristic.

At block 220, the array driver 112 applies a drive voltage between afirst electrode and a second electrode of a display element. The firstelectrode may be one of the movable reflective layers (columnelectrodes) 14 and the second electrode may be one of the row electrodes16 of the interferometric modulator illustrated in FIG. 1. The drivevoltage applied at block 220 may be a static voltage at a bias voltagelevel within the hysteresis window (e.g., 3-7 volts as discussed above),or, alternatively may be a static voltage outside of the hysteresiswindow. By selecting different static voltage levels outside of thehysteresis window, an operational characteristic of the display elementin response to a static, i.e., non-ramped, drive voltage may bedetermined. Operational characteristics that may be affected by thevarious static drive voltage levels applied at the block 220 includeresponse time, maximum sensed current level, amount of stiction, releasevoltage level, actuation voltage level, etc. The static drive voltagedifference applied to the two electrodes at block 220 may be supplied byone or more of the DACs 104 or 108 to the column and/or row electrodes,respectively.

At block 222, the array driver 112 ramps the level of the drive voltagefrom a first level, e.g., the static voltage level applied at block 202,to a second level. The rate of increasing or decreasing ramped voltagelevels (slope of ramp) may be varied for multiple calibration tests. Inthis way, the operational characteristic(s) of the display element maybe determined for the various ramped voltage rates. Operationalcharacteristics that may be affected by the various ramped voltage ratesapplied at the block 222 include response time, maximum current level,amount of stiction, release voltage level, actuation voltage level, etc.The ramped voltage waveform applied at the block 222 may be appliedusing the DDS1 118 illustrated in FIG. 8.

In some embodiments, where the DDS1 118 is faster than the DAC 114, theDDS1 118 is used to supply the variable portion of the signal and theDAC 114 is used to supply the static portion of the signal. In additionin some embodiments, the DDS1 118 may be configured to generate thewaveforms autonomously. In some embodiments, a DDS is configured togenerate a static voltage, and one or more DACs may be used to generatea variable portion of the signal. In some embodiments, one or more DACsor DDS's may be used to generate either or both of the variable andstatic portions of the signal.

The method 200B continues at block 224, where the array driver 112monitors the electrical sensing feedback circuitry (e.g., thetrans-impedance amplifier 120) for the electrical response of thedisplay element. The monitoring functions performed at the step 224 aresimilar to those discussed above in reference to the block 206 of themethod 200A. For example, the trans-impedance amplifier 120 may sense achange in the current to or from the display element, indicating achange in state of the display element. At the block 226, the arraydriver 112 that is receiving the monitored electrical response detects achange of state of the display element. The change of state may be anactuation or a release of the display element. Upon detecting the changeof state of the display element at the block 226, the array driver 112discontinues the ramping of the drive voltage (if a ramped voltage wasapplied at the block 222) at block 228 and the method 200B continues tothe block 230, where information indicative of the drive voltage isstored, e.g., the static voltage level applied at the block 220 and/orthe ramped voltage rate applied at the block 222. In addition, at theblock 230, the array driver 112 stores information indicative of thechange of state of the display element and optionally an operationalcharacteristic of the display element.

The remaining blocks of FIG. 10B are discussed in reference to FIG. 12.In one embodiment, a response time of the display element is monitored.FIG. 12 illustrates an example of a drive voltage waveform for driving adisplay element and the corresponding electrical response sensed indrive circuitry (e.g., the row and/or column electrodes in the row orcolumn switches 110 and 106) connected to the display element, such asmay be used in the methods illustrated in FIGS. 10A and 10B. The exampleof FIG. 12 shows the drive voltage transitioning from a bias voltagelevel where the display element is stable, e.g. in a released state. Attime 320, a static drive voltage is applied (e.g., at the block 220 inthe method 200A) that results in actuation of the display element. Thesensed electrical response, current in this example, exhibits a firstcurrent spike 322 indicating that the voltage across the electrodes haschanged abruptly, followed by a current “bump” 324 which is indicativeof the actuation event. The time between the current spike 322 and thecurrent bump 324 is indicative of the response time (an operationalcharacteristic) of the display element in response to the applied drivevoltage. After the current bump 324 is sensed by the electrical sensingcircuitry, the drive voltage is discontinued at the block 228 (FIG. 10B)and returned to the bias voltage level at 326. When the drive voltage isreduced to the bias voltage level at 326, the sensed electrical responseexhibits another spike 328 indicating that the voltage differencebetween the electrodes of the display element has been abruptly reduced.

The determination of the response time of the display element is anexample of one type of operational characteristic that may be determinedat the block 226 (FIG. 10B) and stored in reference to the appliedvoltage level (the static voltage level and/or the ramped voltage rate)at the block 230. In some embodiments of the display array 202, theresponse time is reduced at higher or more quickly ramped voltage levels(e.g. where a strong electrostatic attraction causes the movable elementto rapidly switch states, where at higher temperatures the springconstant is reduced for the restoring mechanical element, and the like).Other operational characteristics that may be determined and stored inreference to the applied voltage waveforms include maximum sensedcurrent level, amount of stiction, release voltage level, actuationvoltage level, etc. At decision block 234, the array driver 112controlling the calibration method 200B determines if more calibrationcases remain to be tested. If more tests remain, the blocks 220 to 234are repeated for multiple drive periods until no more tests remain andthe method 200B proceeds to block 236.

At the block 236, the array driver 112 determines a drive voltage (thestatic voltage level applied at the block 220 and/or the ramped voltagerate applied at the block 222) based on the information stored at theblock 230 to achieve a desired operational characteristic. For example,it may be desired to achieve a response time below a certain timethreshold in order to more quickly display an image on a display arraycomprising the display elements for which the drive voltages andcharacteristics were calibrated. In another example, it may be desiredto keep the peak current level below a certain value in order to keeptemperatures below a certain level.

In some embodiments, the methods 200A and 200B may be performed inunison. For example, the functions performed at the block 236 may beperformed in conjunction with the method 200A to perform the actuationand release functions of the display element until another calibrationprocess (e.g., the functions at the blocks 220 to 234) is performed at alater time. It should be noted that certain blocks of the methods 200Aand 200B may be omitted, combined, rearranged, or combinations thereof.

The methods illustrated in FIGS. 10A and 10B are examples of methodsthat provide feedback by sensing the electrical response of drivecircuitry, for example, where the feedback detects that a displayelement has been properly actuated or relaxed in response to a givendrive voltage. Another embodiment provides feedback that may be used tosense when a display element has not actuated or released properly. Suchfeedback may be used to adjust the drive voltages to correct theerroneous actuation and/or release states.

FIG. 10C is a flowchart illustrating another method 200C of calibratingdrive voltages for driving a display element including adjusting a drivevoltage based on identifying an error condition when driving the displayelement. In one embodiment, the method 200C can be used for calibratingthe drive voltages of certain display elements for initial testingduring or after manufacture of a display array. This could be done inparallel with the method 200B discussed above. In this embodiment, theprocess 200C can be performed by an external processor connected to thedisplay array, such as a test stand, for example. In another embodiment,the method 200C can be used for adjusting the drive voltage of displayelements during operation upon detecting a failure to actuate a displayelement while the array driver 112 is driving the display array 102 todisplay an image. This later embodiment will be discussed in the exampleshown in FIG. 10C.

The method 200C starts at block 250, where the array driver 112 appliesa drive voltage between a first electrode and a second electrode of adisplay element, wherein the drive voltage is at a level predeterminedto result in the display element being in a first of a plurality ofdisplay states. The first electrode may be one of the movable reflectivelayers (column electrodes) 14 and the second electrode may be one of therow electrodes 16 of the interferometric modulators 12 illustrated inFIG. 1, or vice versa. The drive voltage applied at block 250 may be ata level that has been predetermined to result in actuation of a releaseddisplay element (e.g., a voltage magnitude above the bias voltagerange), a level that has been predetermined to result in release of anactuated display element (e.g., a voltage level lower in magnitude thatthe bias voltage range), or a voltage level that has been predeterminedto keep the display element in the current display state (e.g., avoltage magnitude within the bias voltage hysteresis window as discussedabove).

As discussed above in reference to FIG. 12, release and/or actuation ofa display element can be identified by observing certain electricalresponse characteristics that can be measured by feedback circuitry. Atblock 252, the feedback circuitry is used to measure an electricalresponse of the display element in response to the drive voltage appliedby the drive circuitry at the block 250. The feedback circuitry maycomprise elements such as the trans-impedance amplifier 120 in FIG. 8.At block 254, a processor receives information indicative of theelectrical response measured at the block 252. The array driver 112analyzes the characteristics of the measured electrical response inorder to identify an error in operation of the display element.

An example of a correct actuation and an example of an erroneousactuation of display elements will now be discussed. FIG. 13Aillustrates an example of a drive voltage waveform and correspondingelectrical responses indicative of proper actuation of aninterferometric modulator, such as may be used in the method 200Cillustrated in FIG. 10C. In this example, a released interferometricmodulator 12 is driven to move from a released state to an actuatedstate. The initial voltage difference between the two electrodes is at alevel 331 that is below the actuation voltage threshold level (e.g.,within the bias voltage level) V_(act) in FIG. 13A. At a time point 330,the drive voltage is increased to a level 333 above V_(act) Beginning atthe time point 330, the feedback circuitry measurement, current in thisexample, shows an initial spike 332 followed by a second bump 334. Thesecond bump is indicative that the interferometric modulator 12 hasactuated properly. At a second time point 336, the drive voltage isreduced to the level 331 below V_(act) (within the bias voltage region).At the time point 336, a feedback current exhibits a single spike 338.There is no second bump similar to the bump 334 in the feedback current.This lack of a second bump is indicative that the display elementproperly remained in the actuated state after the time point 336.

FIG. 13B illustrates an example of a drive voltage waveform andcorresponding electrical responses indicative of an example of erroneousactuation of an interferometric modulator 12, such as may be used in themethod illustrated in FIG. 10C. This example is a case where the biasvoltage level is incorrectly calibrated at a level that is outside ofthe bias voltage window. The interferometric modulator 12 may beincorrectly calibrated due to changes in the characteristics of thedisplay element due to age and/or the temperature of the displayelement, for example.

In this example, the initial voltage between the electrodes is at alevel 340 that is below the “bias voltage level”, i.e., the level tosustain the interferometric modulator 12 in the current state. At a timepoint 342, the voltage between the electrodes is increase to a level 344above the actuation voltage level V_(act) in order to actuate theinterferometric modulator 12. The feedback current exhibits a firstspike 346 followed by a second bump 348 that is indicative of a properactuation of the interferometric modulator 12.

At a second time point 350, the voltage between the electrodes isreturned to the initial voltage level 340. The feedback current exhibitsa first spike 352 followed by a second bump 354. This is indicative thatthe interferometric modulator 12 has erroneously released due to thevoltage being lowered to the level 340 that is outside of the biasvoltage window (between the voltage levels V_(rel) and V_(act)). Bydetecting the current bump, the array driver 112 can identify that anerror has occurred at block 254 of the method 200C. Subsequent toidentifying that an error in operation of the interferometric modulator12 has occurred, the array driver 112 can adjust the drive voltage atblock 256 to be at a level greater than V_(rel) and less than V_(act)thereby resulting in a properly tuned interferometric modulator 12 thatremains actuated. The array driver 112 can determine the adjusted drivevoltage level using a method such as discussed above in reference toFIG. 10B.

Skilled technologists will readily be able to use similar methods toidentify proper actuation voltage thresholds of an interferometricmodulator 12. For example, if the interferometric modulator 12 is in theactuated state and the drive voltage applied between the electrodes issupposed to result in releasing the interferometric modulator 12, butthe interferometric modulator 12 does not release, then the array driver112 can adjust the voltage at the block 256 to a lower level until theinterferometric modulator 12 properly releases. In another example, ifthe interferometric modulator 12 is in the released state and thevoltage applied at the block 250 is supposed to actuate theinterferometric modulator 12, but the interferometric modulator 12 doesnot actuate, the array driver 112 can adjust the drive voltage to ahigher value at the block 256 until the interferometric modulator 12actuates properly.

In one embodiment, the method 200C includes an optional block 258 wherethe array driver 112 stores information indicative of the adjusted drivevoltage for later use. The adjusted voltage can be stored withinformation cross-referencing it to a specific interferometric modulator12. The array driver 112 can then use the adjusted value at a later timewhen the specific interferometric modulator 12 is being actuated and/orreleased again. The voltage levels stored at the optional block 258 mayinclude bias voltage levels, release voltage levels and/or actuationvoltage levels, depending on the embodiment.

FIG. 14 is a flowchart illustrating an example of a method 500 fordriving an interferometric modulator 12 and measuring an electricalresponse of the interferometric modulator 12 to determine a drivevoltage to achieve a desired operational characteristic, where the drivevoltage results in a display state transition that is substantiallyundetectable to human vision. The method 500, in one embodiment, enablesdrive voltage levels and/or ramped drive voltage rates (as discussedabove in reference to the methods 200A and 200B of FIGS. 10A and 10B) tobe characterized during operation of the display array 102 in order toadapt to changes in drive voltages quickly. Drive voltage levels maychange due to changing conditions such as age and/or temperature of theinterferometric modulator 12. The method 500 can be performed by thearray driver 112 for controlling the drive circuitry (e.g., the DACs104, 108 and 114, the switches 106 and 110, and the DDS1 118) shown inFIG. 8 to display images on the display array 102. In other embodiments,a processor such as the processor 21 in FIG. 2 can perform the method500.

At block 502, the array driver 112 (FIG. 8) applies a voltage waveformbetween a first electrode and a second electrode of an interferometricmodulator 12, where the voltage waveform alters a state of theinterferometric modulator 12 from a first state to a second state andback to the first state. The voltage waveform applied at the block 502results in the interferometric modulator 12 being altered from areleased state to an actuated state and back to the released state, orvice-versa. In other words, the optical characteristics of the selectedinterferometric modulator 12 (or interferometric modulators 12) ismomentarily disturbed for the measurement of the electrical response ofthe interferometric modulator 12, but the interferometric modulator 12is quickly returned to display the original optical response such that ahuman observer is not aware of the change of state. As noted above, insome embodiments the interferometric modulator 12 can switch states at˜10 kHz, much faster than human vision can detect. Note that when a newimage is “ripped” on the display array (e.g., via a line-at-a-time drivescheme), it is usually desirable that a human user should not be able toperceive the process of one image being overwritten with another. Asuitably fast scan rate or rip rate is chosen for this purpose. When theimage content is changing anyway, a slight momentary disturbance of thecontent for the purpose of measurement can be easily masked from a user.

FIG. 15 illustrates an example of a drive voltage waveform andcorresponding sensed electrical response that may be used at the block502 in the method 500 illustrated in FIG. 15. In this example, asaw-tooth voltage waveform 520 is applied between the electrodes of thedisplay element. In one embodiment, the voltage waveform applied at theblock 502 has a duration from start to finish less than about 400microseconds. However, some embodiments may use voltage wave formshaving end-to-end time durations from about 400 microseconds to about4000 microseconds or larger. The waveform 520 starts with the displayelement in the released state due to the voltage level being at a level522 below the release voltage (V_(rel)) of the display element. Thewaveform 520 then ramps up to a level 524 above the actuation voltagelevel (V_(act)) and then ramps down to a level 526 below the V_(rel)level. Thus the display element transitions from the released state tothe actuated state and back to the released state faster than can bedetected by the user.

Other waveform shapes such as square waves, and sinusoidal waves, forexample, can be applied at the block 502 in the method 500. The specificwaveforms chosen may depend on the specific technology and choice ofalgorithm. The mechanism to apply the waveform may be similar to thosedescribed above in reference to FIG. 8.

While the voltage waveform is being applied at the block 502, thefeedback circuitry (e.g., the trans-impedance amplifier 120) ismonitored at block 504 to measure an electrical response of the displayelement in response to the applied waveform. As discussed above inreference to the methods illustrated in FIGS. 10A, 10B and 10C, anelectrical current of the display element can be monitored to determineif and when an element is released and/or actuated in response to agiven voltage level and/or voltage ramp rate. In FIG. 15, the sensedcurrent typically exhibits a peak 528 when the voltage level exceedsV_(act) and another peak 530 when the voltage declines below V_(rel).The current peak 528 is indicative that the display element hastransitioned from the released state to the actuated state. The currentpeak 530 is indicative that the display element has transitioned back tothe released state. The timing of the sensed current peaks exhibitdifferent characteristics depending on the timing of the actuationand/or release of the display element in response to the applied voltagewaveform.

The feedback circuitry discussed above in reference to FIG. 8 may byused to measure the electrical response at the block 204. The arraydriver 112 receives information indicative of the electrical responsemeasured at the block 504, and at block 506 determines at least oneoperational characteristic of the display element based on the measuredelectrical response. The response time of the display element may bedetermined at the block 506. The response time may vary based on theapplied peak voltage level and/or the voltage ramp rate. In addition,the operational characteristic may include one or more of releasevoltage levels, actuation voltage levels and bias voltage levels. Thesevoltage levels may also vary as a function of temperature of the displayelement, age of the display element, etc.

At optional block 508, the array driver 112 may store informationindicative of the operational characteristic determined at the block 506and store information indicative of the voltage levels applied at theblock 502 to which the operational characteristics correspond. Thevoltage level information stored at the block 508 may include peakvoltage levels, voltage ramp rate, voltage waveform shape, voltagewaveform time duration, and others. The operational characteristicsinformation stored at the block 508 can include response time to actuateor release the display element, actuation voltage levels, releasevoltage levels, bias voltage levels, etc. Release and actuation voltagelevels may also be a function of the ramped voltage rate of thewaveform, and this information may also be stored at the block 508.

After information has been stored at the block 508, the method 500optionally continues to block 510, where the array driver 112 candetermine a drive voltage level and/or ramp rate to apply to a displayelement based on the information stored at the block 508 and a desiredoperational characteristic. In one embodiment, the operationalcharacteristic may simply be actuation or release of the display elementin order to adapt these voltage levels to changing environmentalconditions or age of the interferometric modulator 12. In thisembodiment, the processor or array driver may determine the minimumvoltage amplitude to actuate the display element. In another embodiment,the operational characteristic may be a desired response time. In thisembodiment, the voltage level and/or the voltage ramp rate that bestprovides the desired response time is determined at the optional block510.

The functions performed at the block 502, 504, 506, and optionally 508may be performed on a periodic basis, on a pseudorandom basis, based ona temperature level or change in temperature of the display element ordisplay device, based on the age of the display element, or other basis.

The determination of drive voltage levels at the optional block 510 maybe performed just prior to the array driver 112 signaling the displayelements to display image data during the normal image writing phase.The determination of drive voltage levels at the optional block 510 mayalso be performed on a periodic basis, on a pseudorandom basis, based ona temperature level or change in temperature of the display element ordisplay device, or based on the age of the display element.

Each of the methods discussed above in reference to FIGS. 10A, 10B, 10Cand 14 involve measuring an electrical response of a display element.There are various methods of sensing different portions of a displayarray of display elements. For example, it may be chosen to sense anentire display array in one test. In other words, feedback signals fromall the row electrodes (or column electrodes) may always be electricallyconnected to the trans-impedance amplifier 120 shown in FIG. 8. In thiscase, the timing of the column electrodes being signaled, and the rowsbeing signaled, may be synchronized by the array driver such thatindividual display elements, pixels or sub-pixels (e.g., red, green andblue sub-pixels) may be monitored at certain times. It may also bechosen to monitor or measure one or more specific row or columnelectrodes at one time and optionally switch to monitor other row andcolumn electrodes at other times, and repeating with different rowsand/or columns until the entire array is monitored. Finally, it may alsobe chosen to measure individual display elements and optionally switchto monitor or measure the other display elements until the entire arrayis measured.

In one embodiment, one or more selected row or columns electrodes may bepermanently connected to the stimulus and/or sense circuitry while theremaining row or columns electrodes are not connected to the stimulusand/or sense circuitry. In some embodiments, extra electrodes (row orcolumn) are added to the display area for the purpose of applying thestimulus or sensing. These additional electrodes may or may not bevisible to a viewer of the display area. Finally, another option is toconnect and disconnect the stimulus/drive and/or sense circuitry to adifferent set of one or more row or column electrodes via switches oralternative electrical components.

Embodiments of the systems and methods discussed above may be applied tomonochrome, bi-chrome, or multicolor displays. In some embodiments,groups of pixels for different colors are measured by suitable choice ofrow and column electrodes. For example, if the display uses an RGBlayout where Red (R), Green (G), and Blue (B) sub-pixels are located ondifferent column lines, areas of individual colors may be measured viaapplication of stimulus only to the ‘Red’ columns and sensing on therows. Alternatively, the stimulus may be applied to the rows, but sensedonly on the ‘Red’ columns.

In many display technologies, application of a drive pulse on a givenrow or column may result in undesirable effects on neighboring rows orcolumns. This undesirable effect is commonly called crosstalk. Crosstalkaffects many display technologies including IMOD, LCD and OLED. In oneembodiment, sensing or feedback circuitry is provided to sense existenceof these undesirable effects and compensate. The signal from the area ofinterest can be isolated from the signal or interference from otherregions of a display via various methods.

FIG. 16A is a block diagram illustrating an example of circuitry fordriving an isolated portion of a display array and for sensing anelectrical response of the isolated area. A voltage stimulus V_(in) isapplied to a selected set of column electrodes 540 and a current signalis sensed via a trans-impedance amplifier 542 with low input impedance(Z) from a selected set of row electrodes 544. Thus, a display area 550is sensed. Display areas 555 and 560 are portions of the columnelectrodes 540 and the row electrodes 544, respectively, which are notsensed.

FIG. 16B illustrates a circuit 580 illustrating the electricalrelationship of capacitance of the display area 550 sensed, and thecapacitances of the display areas 555 and 560 not sensed. Capacitor C2represents the capacitance of the display area 555, C3 represents thatof the display area 560 and C1 represents that of the display area 550that is isolated and sensed. The current consumed by C2 is supplied byV_(in) and goes directly to ground. The current through C1, that is thedesired current to be sensed, is also supplied by V_(in), but may beaffected by the capacitance C3 before it reaches the trans-impedanceamplifier 542. However, the current through C1 may be forced to goalmost entirely to the trans-impedance amplifier 542 via choice of anappropriately low input impedance of the trans-impedance amplifier 542as compared to the impedance of the capacitance C3. In this case, thereis substantially no signal current via C3. Thus, from the examplecircuit 580, only the current through C1, the area 555, is sensed by theamplifier. Any area of the display can be selected via correspondingchoice of the row and column electrodes. Note that in the examplecircuitry of FIG. 16B, the remaining electrodes not included in theisolated area are depicted as being connected to ground, however, theycould be connected to any voltage level.

While the above detailed description has shown, described, and pointedout novel features as applied to various embodiments, it will beunderstood that various omissions, substitutions, and changes in theform and details of the device or process illustrated may be madewithout departing from that which has been disclosed. As will berecognized, the present invention may be embodied within a form thatdoes not provide all of the features and benefits set forth herein, assome features may be used or practiced separately from others.

1. A method, comprising: applying a signal between a first electrode anda second electrode of a display element, wherein the signal is at alevel which results in the display element being in a first of aplurality of display states; applying a second signal between the twosaid electrodes to transition the display element from one state toanother, measuring an electrical response of the display element inresponse to the applied second signal; identifying whether or not thedisplay element reached the desired final state based upon the measuredelectrical response; and adjusting the second signal if the displayelement did not reach the desired final state.
 2. The method of claim 1,wherein the first state is an actuated state and the identified error isa failure to relax the display element from the actuated state, themethod further comprising adjusting the second signal to a lower levelin response to identifying the failure to relax the display element. 3.The method of claim 1, wherein the first state is a relaxed state andthe identified error is a failure to actuate the display element, themethod further comprising adjusting the second signal to a higher levelin response to identifying the failure to actuate the display element.4. The method of claim 1, wherein the second signal is a bias voltagelevel which results in the display element remaining in the first state,the first state being an actuated state, the identified error being thedisplay element relaxing, the method further comprising adjusting thesecond signal to a higher level.
 5. The method of claim 1, wherein thesecond signal is a bias voltage level which results in the displayelement remaining in the first state, the first state being a releasedstate, the identified error being actuation of the display element, themethod further comprising adjusting the drive to a lower level.
 6. Themethod of claim 1, further comprising storing information indicative ofthe adjusted drive voltage.
 7. An apparatus, comprising: drive circuitryconfigured to apply a signal between a first electrode and a secondelectrode of a display element, wherein the signal is at a level whichresults in the display element being in a first of a plurality ofdisplay states; feedback circuitry configured to measure an electricalresponse of the display element in response to the applied signal; and aprocessor configured control the drive circuitry, receive informationindicative of the measured electrical response, identify an error inoperation of the display element based upon the measured electricalresponse, and adjust the drive signal of the display element in responseto the identified error.
 8. The apparatus of claim 7, wherein the firststate is an actuated state and the identified error is a failure torelax the display element from the actuated state, and the processor isfurther configured to adjust the signal to a lower level in response toidentifying the failure to release.
 9. The apparatus of claim 7, whereinthe first state is a relaxed state and the identified error is a failureto actuate the display element, and the processor is further configuredto adjust the signal to a higher level in response to identifying thefailure to actuate the display element.
 10. The apparatus of claim 7,wherein the signal is a bias voltage level which results in the displayelement remaining in the first state, the first state being an actuatedstate, the identified error being the display element relaxing, and theprocessor being further configured to adjust the signal to a higherlevel.
 11. The apparatus of claim 7, wherein the signal is a biasvoltage level which results in the display element remaining in thefirst state, the first state being a relaxed state, the identified errorbeing actuation of the display element, the processor being furtherconfigured to adjust the signal to a lower level.
 12. The apparatus ofclaim 7, wherein at least a portion of the feedback circuitry comprisesa portion of the drive circuitry associated with at least one of thefirst and second electrodes.
 13. The apparatus of claim 7, wherein theprocessor is further configured to store information indicative of theadjusted signal.
 14. A display device, comprising: means for applying asignal between a first electrode and a second electrode of a displayelement, wherein the signal is at a level which results in the displayelement being in a first of a plurality of display states; means formeasuring an electrical response of the display element in response tothe applied signal; means for identifying an error in operation of thedisplay element based upon the measured electrical response, and foradjusting the signal in response to the identified error.
 15. A displaydevice comprising: an array of interferometric modulators; drivecircuitry configured to apply a signal between a first electrode and asecond electrode of a display element, wherein the signal is at a levelthat results in the display element being in a first of a plurality ofdisplay states; feedback circuitry configured to measure an electricalresponse of the display element in response to the applied drivevoltage; a processor configured control the drive circuitry, receiveinformation indicative of the measured electrical response, identify anerror in operation of the display element based upon the measuredelectrical response, and adjust the signal in response to the identifiederror; and a memory device configured to communicate with the processor.16. The display device of claim 15, further comprising a controllerconfigured to send at least a portion of the image data to the drivecircuitry.
 17. The display device of claim 15, further comprising animage source module configured to send the image data to the processor.18. The display device of claim 17, wherein the image source modulecomprises at least one of a receiver, transceiver, and transmitter. 19.The display device of claim 15, further comprising an input deviceconfigured to receive input data and to communicate the input data tothe processor.